Combustion Analysis Simulator

About This Simulator

Combustion analysis is the process of measuring the composition of exhaust gases to determine how efficiently fuel is being burned. This simulator estimates combustion excess air and stack (flue-gas) heat loss using gas measurements and the empirical Siegert method.

Measuring CO₂ and O₂ concentrations is essential because they indicate the amount of air supplied to the combustion process relative to the theoretical minimum required. Excess air is the extra air provided to ensure complete combustion. While necessary, too much excess air lowers boiler or furnace efficiency by carrying sensible heat out through the stack. The flue-gas temperature represents the thermal energy leaving the system. The Siegert method provides a rapid engineering estimate of these stack heat losses based on measured gas composition and temperatures.

How to Use the Simulator

1. Select the fuel: Choose from Natural Gas, Fuel Oil, LPG, or define custom parameters.
2. Select calculation mode: Choose whether you are entering measured CO₂ or measured O₂.
3. Enter the measured gas concentration: Input the CO₂ or O₂ percentage obtained from the flue-gas analyzer.
4. Enter flue-gas temperature: Input the temperature of the exhaust gases leaving the stack.
5. Enter ambient temperature: Input the combustion-air (ambient) temperature entering the burner.
6. Run the calculation: Review the estimated dilution, excess air, heat loss, and combustion efficiency.

Engineering Background

Excess Air

Theoretical or stoichiometric air is the exact amount of air required to completely burn a specific quantity of fuel. In practice, achieving perfect mixing is impossible, so burners are supplied with actual combustion air, which is greater than the theoretical requirement.

Excess air is defined as the percentage of air supplied above the theoretical minimum:

Instead of manually calculating mass flow rates, this simulator determines excess air directly from the flue-gas composition (CO₂ or O₂ concentrations). Proper excess air control is vital: too little causes incomplete combustion (producing CO and soot), while too much dilutes the heat and wastes energy.

CO₂ Method

When a fuel burns completely with exact stoichiometric air, the resulting CO₂ concentration in dry exhaust gases reaches its maximum theoretical CO₂ (CO₂max). If excess air is introduced, this extra air dilutes the combustion gases. Therefore, more excess air leads to greater dilution and a lower measured CO₂.

The simulator calculates the dilution factor (D) and excess air using:

Make clear that this is a combustion-gas analysis approach based on the relationship between measured and theoretical maximums.

O₂ Method

Alternatively, the residual oxygen in the exhaust gas directly indicates the amount of excess air. The simulator relies on the assumption that dry atmospheric air contains approximately 21% oxygen by volume:

This method is widely preferred in modern analyzers because oxygen measurement is less sensitive to fuel composition variations than CO₂ measurement.

Siegert Method

The Siegert method estimates the sensible heat loss (q) carried away by the hot exhaust gases. It relates the temperature difference between the flue gas (Tg) and combustion-air ambient temperature (Ta) to the CO₂ concentration:

Here, K is an empirical constant specific to the fuel type. This equation provides the percentage of total fuel energy lost to the stack under the assumptions of the method.

Interpreting the Results

Excess Air

• Low excess air: Risks incomplete combustion, soot formation, and safety hazards.
• Appropriate excess air: Varies by fuel and burner design, but typically ensures complete combustion while minimizing dilution losses.
• Excessively high excess air: Decreases efficiency because a large volume of nitrogen and extra oxygen is needlessly heated and discharged through the stack.

Siegert Heat Loss

A higher calculated stack loss generally indicates more energy is leaving with the flue gases. This loss increases with high flue-gas temperatures, low CO₂ concentrations, and elevated excess air.

Estimated Efficiency

The simulator derives combustion efficiency (η) directly from the Siegert stack-loss estimate:

This value represents the efficiency estimate associated strictly with the Siegert stack-loss model. It should not automatically be interpreted as a complete boiler efficiency including every possible loss. This distinction is important.

Engineering Applications

This type of calculation is widely useful in:

• Industrial boilers and furnaces
• Burner tuning
• Boiler commissioning
• Combustion diagnostics
• Energy-efficiency analysis
• Maintenance and performance monitoring

Assumptions and Limitations

To ensure engineering credibility, users must understand the limitations of this model:

• The simulator uses standard parameters for the selected fuel model.
• The combustion analysis is based entirely on the entered gas composition.
• The Siegert method is an empirical/engineering estimation method.
• The estimated efficiency is associated exclusively with stack/flue-gas loss represented by the model.
• Real boiler/furnace efficiency may also depend on other losses not calculated here, such as:
  • Radiation
  • Unburned fuel or incomplete combustion (e.g. CO formation)
  • Ash-related losses where applicable
  • Other heat losses through the boiler shell

Example

Consider a natural gas boiler being tested during maintenance. The following data is recorded:

• Fuel: Natural Gas (CO₂max = 11.7%, K = 0.39)
• Measured CO₂: 9.5%
• Flue-gas temperature: 220 °C
• Combustion-air temperature: 20 °C

Frequently Asked Questions